An ultrasonic imaging apparatus transmits/receives ultrasonic waves to/from an object being examined through an ultrasonic probe, and reconstructs and displays an ultrasonic image (for example, grayscale tomographic image, a color blood stream image) of the object being examined on the basis of a reflection echo signal output from the ultrasonic probe. A noninvasive and real-time diagnosis of an image pickup site can be performed on the basis of such an ultrasonic image.
In the ultrasonic imaging apparatus as described above, three-dimensional tomographic image data (hereinafter referred to as tomogram volume data) and three-dimensional Doppler image data (hereinafter referred to as Doppler image volume data) are obtained on the basis of the reflection echo signal output from the ultrasonic probe, and a tissue projection image generated from the obtained tomogram volume data and a Doppler projection image generated from the Doppler image volume data are combined with each other and displayed. On the basis of this composite image, for example, the positional relationship between a blood vessel and tissues around the blood vessel is grasped, and the state of a nutrient vessel of a cancer tissue is recognized from the positional relationship, whereby it is judged whether the cancer tissue is protopathic or metastatic (for example, see Patent Document 1).    Patent Document 1: see U.S. Pat. No. 6,280,387
However, when a tissue projection image and a Doppler projection image are combined and displayed as in the case of the Patent Document 1, an image at the point-of-view side of a diagnosing person is preferentially displayed, and thus a part of the Doppler projection image may hide in the tissue projection image. Accordingly, in order to surely grasp the stereoscopic positional relationship between the blood vessel and the tissues around the blood vessel (for example, the penetration state of the blood vessel through the tissues), it is necessary to carry out an operation of rotating the display angle of the composite image or remove unnecessary image data from the composite image, and thus there is a problem that this device is inconvenient.
Furthermore, when the tissue projection image and the Doppler projection image are displayed in superposition with each other while varying the composition rate between the tissue projection image and the Doppler projection image, the brightness of the respective images is equally changed in connection with the change of the composition rate, and thus it is difficult to grasp, from the displayed composite image, the stereoscopic positional relationship between the blood vessel and the tissue around the blood vessel, for example, the overlap condition of the respective images when viewed from the projection direction, depth feel in the projection direction in the overlap area, etc.